feat(forum): multi-gateway cashier for pay-visibility topics

Replace balance-only AJAX topic purchase with AKSRT_Topic_Pay REST
cashier (resource orders + Payment_Gateway), reuse cms-download modal,
and unlock on aksrt_order_paid.

functions.php

@@ -177,6 +177,9 @@ if (class_exists('AKSRT_Download')) {
 if (class_exists('AKSRT_Content_Pay')) {
     AKSRT_Content_Pay::get_instance()->init();
 }
+if (class_exists('AKSRT_Topic_Pay')) {
+    AKSRT_Topic_Pay::get_instance()->init();
+}
 if (class_exists('AKSRT_Vip_Service')) {
     AKSRT_Vip_Service::get_instance()->init();
 }